The use of nondestructive inspection techniques to evaluate the condition of solid structures is well known, particularly in those situations where ultrasonic sound beams are directed beneath a solid surface for the detection of cracks or voids therein. In performing ultrasonic inspection beneath the surface of an object, a beam of ultrasonic sound energy is directed downward into the solid body such that the transmitted beam will pass through a zone of interest in which the presence of any flaws is to be determined. Should a flaw exist within this zone of interest, the ultrasonic sound beam will be reflected by the flaw and be detected by a properly positioned directional receiver or microphone located outside of the solid body. By properly positioning the sound transmitter and directional receiver, it is possible to restrict the size of the zone of interest in which the presence of flaws will cause ultrasonic sound energy to be reflected to the receiver.
One industry in which this type of limited inspection is desirable is the nuclear power industry.. Nuclear reactor vessels are typically fabricated from a base material such as carbon steel and coated with a cladding of stainless or other temperature and radiation resistant material. For a clad pressure vessel, the presence of any flaws or cracks in the base material or cladding is detrimental to the functionality and safety of the vessel. As a reflection of the importance of this factor, governmental regulatory agencies charged with the supervision of the nuclear power industry have required that inspection techniques used not only during the construction of the vessels, but also for the in-service inspection of operating reactors, include a thorough inspection of the region beneath inner cladding in the base material, as well as the cladding itself.
A number of ultrasonic inspection devices have been developed and are in use which concentrate on the zone of interest which includes the cladding and the base material just beneath it. As discussed above, this limitation is achieved by properly positioning the ultrasonic sound transmitter and directional receiver in relation to the solid body undergoing inspection. As the greater part of the clad vessel is of a regular geometric shape, i.e., cylindrical or spherical, it has been relatively straightforward to develop the ultrasonic inspection devices for detecting flaws. However, there exists one portion of the reactor vessel wherein the geometric shape is not constant. This area is the junction between the coolant flow nozzles and the reactor vessel. This junction, in reality the intersection of a large vertical cylinder and a small horizontal cylinder, produces a saddle-shaped corner region wherein the angle of the corner can vary from 90.degree. to approximately 130.degree., dependent upon the radial displacement about the axis of the small horizontal cylinder, or coolant nozzle.
Due to the variable geometry. of this section, inspection techniques heretofore used or proposed have succeeded in only approximate inspections of this potentially critical region, or have required the use of multiple ultrasonic transmitters and directional receivers in order to thoroughly perform the desired inspection. See assignee's co-pending application, Ser. No. 503,978, filed June 13, 1983 which discloses a corner region inspection device utilizing a plurality of ultrasonic transmitters and directional receivers for accommodating the variable geometry of the nozzle corner region.
While multiple transmitters and receivers have proved effective, the use of such systems requires the operator to continually monitor the radial displacement of the inspection device for the purpose of determining which transmitter-receiver combination is currently operable for inspecting the cladding and interface. Moreover, those devices currently proposed or in use must be moved along a saddle-shaped path which corresponds to the shape of the nozzle-vessel intersection. Such movement requires relatively complex control of the arm mechanism which remotely manipulates the inspection devices within the reactor vessel. What is required is an ultrasonic inspection device for examining a zone of interest within the variable geometry. corner region, such as is found in a nuclear reactor vessel, which is self-aligning so as to not reouire exact or very complex motion of the instrument manipulator arm.